In magnetic resonance imaging, an object to be imaged as, for example, a body of a human subject, is exposed to a strong, substantially constant static magnetic field. The static magnetic field causes the spin vectors of certain atomic nuclei within the body to randomly rotate or “precess” around an axis parallel to the direction of the static magnetic field. Radio frequency excitation energy is applied to the body, and this energy causes the precessing automatic nuclei to rotate or “precess” in phase and in an excited state. As the precessing atomic nuclei relax, weak radio frequency signals are emitted; such radio frequency signals are referred to herein as magnetic resonance signals.
Different tissues produce different signal characteristics. Furthermore, relaxation times are the dominant factor in determining signal strength. In addition, tissues having a high density of certain nuclei will produce stronger signals than tissues with a low density of such nuclei. Relatively small gradients in the magnetic field are superimposed on the static magnetic field at various times during the process so that magnetic resonance signals from different portions of the patient's body differ in phase and/or frequency. If the process is repeated numerous times using different combinations of gradients, the signals from the various repetitions together provide enough information to form a map of signal characteristics versus location within the body. Such a map can be reconstructed by conventional techniques known in the magnetic resonance imaging art, and can be displayed as a pictorial image of the tissues as known in the art.
The magnetic resonance imaging technique offers numerous advantages over other imaging techniques. MRI does not expose either the patient or medical personnel to X-rays and offers important safety advantages. Also, magnetic resonance imaging can obtain images of soft tissues and other features within the body which are not readily visualized using other imaging techniques. Accordingly, magnetic resonance imaging has been widely adopted in the medical and allied arts.
As discussed above, MRI images are taken by acquiring multiple sets of MRI data and compounding those sets to produce a single MRI image. For many years it has been desirable to collect each data set when a predetermined physiological event occurs. The event may be a respiratory event, a muscle movement, a cardiac motion, or any other physiological event. In that regard, in MRI applications, it is often necessary to have the ability to accurately gate the MRI system so the data may be collected when the physiological event occurs.
In some instances, of particular interest in MRI applications is the ability to time the acquisition of data to one point during the cardiac cycle. The most common point used is ventricular contraction, as visualized on the electrocardiogram (ECG) as the QRS wave. The contraction also produces a pressure pulse that is transmitted throughout the whole arterial system of a patient. Using the ECG as a gating technique has been the standard used for years. Nevertheless, there are several disadvantages inherent in using ECG in MRI gating. First, there are fast switching magnetic gradients that induce currents in wires that connect to ECG sensors. Similarly there are induced currents in the skin of the subject that also interfere with the acquisition of the ECG. And finally, there are safety concerns about wires that are in contact with the patient as currents might be induced in those wires by the MRI systems.